The present invention relates to a document reading apparatus having a document reading system while a document is moving and/or a scanning exposure reading system while a document is stationary, and further, the present invention relates to an image forming apparatus provided with the document reading apparatus.
The document reading apparatus in the conventional image forming apparatus has a shading correction function, and as the shading correction function, a system by which a predetermined area of a reference density plate is read out, and the shading correction data is averaged, is adopted, however, because also a dust adhered to the reference density plate is averaged, there is a problem that it is difficult to conduct a correct shading correction.
Therefore, in order to simply and surely conduct the proper shading correction, the following method is well known. FIG. 8(a) is an illustration of finding method of the shading correction data, and FIG. 8(b) is a view showing the abnormality of the shading correction data. As shown in FIGS. 8(a) and 8(b), the collection of the shading correction data F1 and F2 is conducted at two collection positions of the reference density plate, and only the data whose brightness value is higher is selected, and is made the shading correction data F.
However, because two collection positions of the reference density plate are fixed, as shown in FIG. 8(b), for example, when a dust adhered onto the collection position of the shading correction data F1, the brightness value is quickly lowered at the dust adhered position, and the data difference between the shading correction data F1 and F2 is increased at the lowered position, and there is a problem that the correction is not correctly conducted.
Further, FIG. 9 is a view showing an influence on the unnecessary light at the time of the shading correction data collection. In FIG. 9, an exposure unit 16 of a document reading apparatus is composed of the moving body 160, the first mirror 161, and the light source 162, and the reference density plate 15 for the shading correction is positioned and fixed on the upper surface of a slit exposure glass 12 inside a document guide member 120. The reflection light of the reference density plate 15 illuminated by the light source 162 is incident on a CCD image sensor, not shown, through the exposure unit 16, and is photo-electrically converted by this CCD image sensor. In two collection positions 15a and 15b of the reference density plate 15, for example, when the collection position 15a is deviated to a collection position 151a shown by a solid line, an unnecessary light L1 is generated, and an error is generated in the shading correction data, and when the collection position 15b is deviated to a collection position 151b shown by a dotted line, the unnecessary light L2 is generated, and the error is generated in the shading correction data. However, even when the position dislocation of the collection position is generated, there is also a problem that the collection position dislocation cannot be previously detected.